1. Field of the Invention
The invention relates to a loop antenna; more particularly, the invention relates to a loop antenna with an adjustable bandwidth.
2. Description of Related Art
Research and development of communication devices are often conducted according to the market demands, and different types of the communication devices are presented to the public step by step. For instance, since a wireless fidelity (Wi-Fi) function has already become indispensable for normal users, a new communication device featuring Wi-Fi is often presented at first; in a later stage, the same type of communication device that further supports global system for mobile communication (GSM) or long term evolution (LTE) may then be developed. In theory, when the device featuring Wi-Fi is designed, the space for placing antennas with low frequency bands (e.g., GSM or LTE) may be kept in advance, so as to save the time and costs on subsequent re-molding and re-design.
The space for placing antennas with low frequency bands may sometimes not be kept in some types of devices. In this case, it is rather difficult for a designer to design antennas. For instance, the space capable of placing an antenna may have other circuits or metal and is not completely available, and a printed circuit board with high-speed signal lines or any other metal devices may be extremely close to the antenna, e.g., the distance therebetween may be less than 5 mm. Under these circumstances, the types of antennas become crucial, and a loop antenna, for instance, is not apt to be affected by surroundings.
Generally, the length of the antenna is determined according to the minimum frequency required by the antenna. For instance, if the desired frequency is 824 MHz, the required wavelength is approximately 0.364 meters because the light velocity (3×108 meters/seconds) is calculated by multiplying the frequency of light (8.24×108 MHz) by the wavelength. Since the required length of the loop antenna is half the wavelength, i.e., 0.182 meters, the required length of the loop antenna in the limited space within the communication device needs to satisfy 0.182 meters. As a result, the loop antenna within the communication device is often designed to be bent or surround the insulation elements in the communication device, such that the loop antenna may occupy less space.
FIG. 1 is a schematic view illustrating a conventional loop antenna. With reference to FIG. 1, the conventional loop antenna 10 includes a feed 12, a ground 14, and a circular loop 16 that is connected to the feed 12 and the ground 14. After the sample loop antenna 10 is made, the circular loop 16 can be barely adjusted. Even though the bandwidth corresponding to the conventional loop antenna 10 is different from the originally designed bandwidth, it is rather difficult to rectify this deficiency.